Light olefins, defined herein as ethylene, propylene, butylene and mixtures thereof, serve as feeds for the production of numerous important chemicals and polymers. Typically, light olefins are produced by cracking petroleum feeds. Because of the limited supply of competitive petroleum feeds, the opportunities to produce low cost light olefins from petroleum feeds are limited. Efforts to develop light olefin production technologies based on alternative feeds have increased.
An important type of alternate feed for the production of light olefins is oxygenate, such as, for example, alcohols, particularly methanol and ethanol, dimethyl ether, methyl ethyl ether, diethyl ether, dimethyl carbonate, and methyl formate. Many of these oxygenates may be produced by fermentation, or from synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials, including coal, recycled plastics, municipal wastes, or any organic material. Because of the wide variety of sources, alcohol, alcohol derivatives, and other oxygenates have promise as an economical, non-petroleum source for light olefin production.
The catalysts used to promote the conversion of oxygenates to olefins are molecular sieve catalysts. Because ethylene and propylene are the most sought after products of such a reaction, research has focused on what catalysts are most selective to ethylene and/or propylene, and on methods for increasing the life and selectivity of the catalysts to ethylene and/or propylene.
The conversion of oxygenates to olefins generates and deposits carbonaceous material (coke) on the molecular sieve catalysts used to catalyze the conversion process. Over accumulation of these carbonaceous deposits will interfere with the catalyst's ability to promote the reaction. In order to avoid unwanted build-up of coke on the molecular sieve catalyst, the oxygenate to olefin process incorporates a second step comprising catalyst regeneration. During regeneration, the coke is removed from the catalyst by combustion with oxygen, which restores the catalytic activity of the catalyst. The regenerated catalyst then may be reused to catalyze the conversion of oxygenates to olefins.
Typically, oxygenate to olefin conversion and regeneration are conducted in two separate vessels. The coked catalyst is continuously withdrawn from the reaction vessel used for conversion to a regeneration vessel and regenerated catalyst is continuously withdrawn from the regeneration vessel and returned to the reaction vessel for conversion.
U.S. Pat. No. 4,547,616 to Avidan et al., incorporated herein by reference, discloses a process for converting oxygenates to lower olefins by operating a fluidized bed of zeolite catalyst, e.g. ZSM-5, whose activity is controlled to produce a product having propane:propene molar ratio ranging from 0.04:1 to 0.1:1.
U.S. Pat. No. 4,873,390 to Lewis et al., incorporated herein by reference, teaches conversion of a feedstock, e.g., alcohols, to a product containing light olefins over a silicoaluminophosphate having pores with diameters of less than 5 Angstroms, wherein carbonaceous deposit material is formed on the catalyst. The catalyst is treated to form a partially regenerated catalyst having from 2 to 30 wt. % of the carbonaceous deposit material, with a preferred range between 4 and 20 wt. %.
U.S. Pat. No. 6,137,022 to Kuechler et al., incorporated herein by reference, discloses a method of increasing selectivity of a reaction to convert oxygenates to olefins by converting the feedstock in a reaction zone containing 15 volume percent or less of a catalyst comprising a silicoaluminophosphate molecular sieve material, and maintaining conversion of the feedstock between 80% and 99% under conditions effective to convert 100% of the feedstock when the reaction zone contains at least 33 volume percent of the molecular sieve material.
U.S. Pat. No. 6,023,005 to Lattner et al., incorporated herein by reference, discloses a method of producing ethylene and propylene by catalytic conversion of oxygenate in a fluidized bed reaction process which utilizes catalyst regeneration. The process maintains a portion of desired carbonaceous deposits on the catalyst (wt. %) by removing only a portion of the total reaction volume of coked molecular sieve catalyst and totally regenerating only that portion of catalyst, which is then mixed back with the unregenerated remainder of catalyst. The resulting catalyst mixture contains 2–30 wt % carbonaceous deposits.
S. Soundararajan et al., “Modeling of methanol to olefins (MTO) process in a circulating fluidized bed reactor”, Fuel 80 (2001), 1187–1197 at 1192–93, discuss the effect of coke content (wt. %) on product selectivities from pure methanol using SAPO-34 catalyst. Although a relationship between wt. % carbon on catalyst and selectivity for primary olefins has been observed, it can vary considerably from one catalyst to another, even for molecular sieve materials having the same structure.
Stephen Wilson et al., “The characteristics of SAPO-34 which influence the conversion of methanol to light olefins”, Microporous and Mesoporous Materials 29(1999) 117–126, describe the relationship between acid-site strength and density on methanol conversion to light olefins over chabazite structure types (SAPO-34 and SSZ-13) in terms of determining which catalyst was the most resistant to coking at one acid-site per chabazite cage.
Ivar M. Dahl et al., “Structural and chemical influences on the MTO reaction: a comparison of chabazite and SAPO-34 as MTO catalysts”, Microporous and Mesoporous Materials 29(1999) 185–190, describe the effect of relationship between acid-site strength and susceptibility of chabazite and SAPO-34 to deactivation for oxygenate conversion at high space velocities to avoid excess catalyst activity causing undesired secondary reactions.
It would be desirable to provide a process for making olefins from oxygenate which maximizes primary olefin yield, especially light olefin yield (ethylene and propylene), for a wide variety of catalysts.